I can't speak specifically to the application you've mentioned, but there have been a few issues with people getting mics and headsets to work well with Parallels. Do a search for "headset mic" on the
Parallels Desktop for Mac Forum and you'll find a number of hits and some ways around the problems people have had.
My gut feeling is that either Parallels or Boot Camp will work equally well for what you are thinking of. If you do go with Parallels you can always try the 15-day trial key before buying (nothing lost if it doesn't work). Also, Parallels and Boot Camp coexist with each other well, so you can have both and eventually choose whichever is working best for you. Having both just chews up some extra disk space as you have two installations of Windows, etc.
Intel Mini 1.66 GHz, Mac Pro 2.66 GHz Mac OS X (10.4.8)
Vista and XP running under Parallels